CD63 and breast cancer: To investigate the potential exosomal miRNA from breast cancer, we first used a cell line model and selected the highly metastatic breast cancer cell line MDA-MB-231 to establish a stable cell line, MDA-MB-231-CD63-RFP (231-CD63), which expresses the exosomal marker CD63 tagged with red fluorescent protein (RFP) to track exosomes.